Cellular and satellite communication systems are well known in the art for providing communications links between mobile telephone users and stationary users or other mobile users. These communications links may carry a variety of different types of information, including voice, data, video and facsimile transmissions. In typical cellular systems, wireless transmissions from mobile users are received by local, terrestrial based, transmitter/receiver stations. These local base stations or "cells" then retransmit the mobile user signals, via either the local telephone system or the cellular system, for reception by the intended receive terminals.
Many cellular systems rely primarily or exclusively on line-of-sight communications. In these systems, each local transmitter/receiver has a limited range, and consequently, a large number of local cells may be required to provide communications coverage for a large geographic area. The cost associated with providing such a large number of cells may prohibit the use of cellular systems in sparsely populated regions and/or areas where there is limited demand for cellular service. Moreover, even in areas where cellular service is not precluded by economic considerations, "blackout" areas often arise in terrestrial based cellular systems due to local terrain and weather conditions.
As such, it has been proposed to provide a combined cellular/satellite communications network that integrates a limited terrestrial based cellular network with a satellite communications network to provide communications for mobile users over a large geographical area where it may be impractical to provide cellular service. In the proposed system, terrestrial based cellular stations would be provided in high traffic areas, while an L-Band satellite communications network would provide service to remaining areas. In order to provide both cellular and satellite communications, the user terminal handsets used with this system would include both a satellite and a cellular transceiver. Such a combined system could provide full communications coverage over a wide geographic area without requiring an excessive number of terrestrial cells.
In this proposed system, which is known as the Asian Cellular Satellite System, the satellite network would be implemented as one or more geosynchronous satellites orbiting approximately 22,600 miles above the equator. These satellites could provide spot beam coverage over much of the far east, including China, Japan, Indonesia and the Philippines. In this system, signals transmitted to the satellite will fall within the 1626.5 MHz to 1660.5 MHz transmit frequency band, and the signals transmitted from the satellite will fall within the 1525 MHz to 1559 MHz receive frequency band.
While integrating satellite and cellular service together in a dual-mode system may overcome many of the disadvantages associated with exclusively terrestrial based cellular systems, providing dual-mode user terminal handsets that meet consumer expectations regarding size, weight, cost, ease of use and communications clarity is a significant challenge. Consumer expectations relating to such physical characteristics and communications performance of handheld mobile phones have been defined by the phones used with conventional cellular systems, which only include a single transceiver that communicates with a cellular node which typically is located less than 20 miles from the mobile user terminal. By way of contrast, the handheld user terminals which will be used with the Asian Cellular Satellite System must include both a cellular and a satellite transceiver. Moreover, the large free space loss associated with the satellite communications aspect of the system may significantly increase the power and antenna gain which must be provided by the antenna for the satellite transceiver on the user terminal handset, as the signals transmitted to or from the satellites undergo a high degree of attenuation in traveling the 25,000 or more miles that typically separates the user handset from the geosynchronous satellites.
Furthermore, the satellite aspects of the network also may impose additional constraints on the user terminal handsets. For instance, the satellite transceiver provided with the user terminal handset preferably should provide a quasi-hemispherical antenna radiation pattern, in order to avoid the need to track a desired satellite. Additionally, the antenna which provides this quasi-hemispherical radiation pattern should transmit and receive a circularly polarized waveform, so as both to minimize the signal loss resulting from the arbitrary orientation of the satellite antenna on the user terminal with respect to the satellite and to avoid the effects of Faraday rotation which may result when the signal passes through the ionosphere. Moreover, the satellite antenna on the handheld transceiver should also have a low front-to-back ratio and low gain at small elevation angles in order to provide a low radiation pattern noise temperature.
In addition to the above constraints, it is also preferable that the handset satellite transceiver be capable of operating over the full extent of the transmit and receive frequency bands associated with the satellite network. The operating frequency band of the Asian Cellular Satellite System, however, is as large as any communications bandwidth associated with user terminal antenna systems employed in various prior art L-Band satellite communications systems. Moreover, as discussed above, the satellite network transmits signals in one frequency band (the transmit frequency subband) and receives signals in a separate frequency band (the receive frequency subband) in order to minimize interference between the transmit and receive signals. Thus the satellite transceiver on the user handset preferably provides an acceptable radiation pattern across both the transmit and receive frequency subbands.
In light of the above constraints, there is a need for handheld satellite transceivers, and more specifically, antenna systems for such transceivers, capable of transmitting and receiving circularly polarized waveforms which provide a relatively high gain quasi-hemispherical radiation pattern over separate, relatively broadband, transmit and receive frequency subbands. Such an antenna system preferably would be capable of receiving signals from, or transmitting signals to, satellites which may be located anywhere in the hemisphere. Moreover, given the handheld nature of the user terminals and consumer expectations of an antenna which is conveniently small for ease of portability, the satellite antenna system capable of meeting the aforementioned requirements should fit within an extremely small physical volume. These user imposed size constraints may also place limitations on the physical volume required by the antenna feed structure and any matching, switching or other networks required for proper antenna operation. Thus, for instance, in the Asian Cellular Satellite System, the satellite network link budgets require the satellite antenna system on the handheld phone to be capable of providing a net gain of at least 2 dBi over all elevation angles exceeding 45.degree., where the net gain is defined as the actual gain or "directivity" provided by the antenna minus matching, absorption or other losses incurred in the antenna feed structure. Additionally, the antenna must also have an axial ratio of less than 3 dB while providing good front to back ratio over the entire receive frequency subband. These performance characteristics must be provided by an antenna which, along with any associated impedance matching circuits or other components, fits within a cylinder 13 centimeters in length and 13 millimeters in diameter.
Helix antennas, and in particular, multifilar helix antennas, are relatively small antennas that are well suited for various applications requiring circularly polarized waveforms and a quasi-hemispherical beam pattern. A helix antenna is a conducting wire wound in the form of a screw thread to form a helix. Such helix antennas are typically fed by a coaxial cable transmission line which is connected at the base of the helix. A multifilar helix antenna is a helix antenna which includes more than one radiating element. Each element of such a multifilar helix antenna is generally fed with an equal amplitude signal that is separated in phase by 360.degree./N, where N is the number of radiating antenna elements. As the phase separation between adjacent elements varies from 360.degree./N, the antenna pattern provided by the multifilar helix antenna tends to degrade significantly. Accordingly, the feed structure which couples the signals between the elements of a multifilar helix antenna and the transmitter/receiver preferably introduces minimal or no phase distortions so that such degradation of the antenna pattern is minimized or prevented.
A common type of multifilar helix antenna is the quadrifilar helix. The quadrifilar helix antenna is a circularly polarized antenna which includes four orthogonal radiating elements arranged in a helical pattern (which may be fractional turn), which are excited in phase quadrature (i.e., the radiated energy induced into or from the individual radiating elements is offset by 90.degree. between adjacent radiating elements).
Quadrifilar helix antennas can be operated in several modes, including axial mode, normal mode or a proportional combination of both modes. To achieve axial mode operation, the axial length of each antenna element is typically several times larger than the wavelength corresponding to the center frequency of the frequency band over which the antenna is to operate. Operated in this mode, a quadrifilar helix antenna can provide a relatively high gain radiation pattern. However, such a radiation pattern is highly directional (i.e., it is not quasi-hemispherical) and hence axial mode operation is typically not appropriate for satellite communications terminals that do not include means for tracking the satellite.
Operated in the normal mode, each helix of a quadrifilar helix antenna is typically balun fed at the top, and the helical arms are typically of resonant length (i.e., 1/4.lambda., 1/2.lambda., 3/4.lambda. or .lambda. in length, where .lambda. is the wavelength corresponding to the center frequency of the frequency band over which the antenna is to operate). These elements are wound on a small diameter with a large pitch angle. In this mode, the antenna typically provides the quasi-hemispherical radiation pattern necessary for mobile satellite communications, but unfortunately, the antenna only provides this gain over a relatively narrow bandwidth situated about the resonant frequency. Moreover, the natural bandwidth of the antenna is proportional to the diameter of the cylinder defined by the quadrifilar helix antenna, and thus, all else being equal, the smaller the antenna the smaller the operating bandwidth. As discussed above, certain emerging cellular and satellite phone applications have relatively large transmit and receive operating bandwidths. These bandwidths may approach or even exceed the bandwidth provided by quadrifilar helix antennas operated in normal mode, and this is particularly true where other system requirements significantly restrict the maximum diameter of the antenna.
In addition to the above-mentioned bandwidth limitations associated with quadrifilar helix antennas, the bandwidth over which these antennas may effectively operate may also be limited by power transfer considerations. Specifically, in operation, it is necessary to transfer electrical signals between a transmitter/receiver and the quadrifilar helix antenna. However, such power transfer typically is not lossless due to reflections which arise as a result of imperfect impedance matching between the source and the load. If large enough, the reflected power loss, which may be expressed in terms of voltage standing wave ratio ("VSWR"), may prevent the communications system from meeting its link budgets. By way of example, for the Asian Cellular Satellite System, system link budgets require that the voltage standing wave ratio, as measured at the output of the handset transmitter/receiver, be less than 1.5.
While it often is possible to match the input impedance of the quadrifilar helix antenna to the impedance of the interconnecting transmission line(s) from the transmitter/receiver, such a match will only occur over a small frequency range as the input impedance of a quadrifilar helix antenna varies significantly with frequency. Accordingly, even if a perfect match (i.e., VSWR=1.0) is not required, an acceptable match will typically still only be achievable over some finite bandwidth. This bandwidth may be less than the operating bandwidth required by emerging cellular and satellite phone applications. As such, impedance mismatches may also serve to limit the effective bandwidth of quadrifilar helix antenna systems.
Quadrifilar antennas have previously been used in a number of mobile L-Band satellite communication applications, including INMARSAT, NAVSTAR, and GPS. However, nearly all these prior art antennas were physically much too large to satisfy the size requirements of emerging satellite phone applications. Moreover, these prior art antennas also generally do not meet the size constraints imposed by these emerging applications while also providing the gain, axial ratio, noise temperature, front-to-back ratio and broadband performance that are required by these emerging applications. Accordingly, a need exists for a new, significantly smaller, satellite phone antenna system that is capable of providing a quasi-hemispherical antenna pattern with positive gain over widely separated, relatively broadband, transmit and receive frequency subbands.